The present invention relates to binoculars.
When an object at infinity is observed by a pair of binoculars, a field of view observed by a left eye of an observer and a field of view observed by the right eye substantially overlap each other, and a single field of view is observed when the observer observes the binoculars with both eyes. When an object at a relatively short distance of several meters or less is observed with the binoculars, only a part of the field of view for each of the right eye and left eye overlaps each other, and the observer feels difficulty in observing such an object. This is because, in binoculars, the optical axes of left and right objective lenses are generally fixed to be parallel to each other since the binoculars are generally designed to observe an object located within a range from several tens of meters to infinity. If an object at a short distance is observed with such binoculars, a remarkable discrepancy arises between a focusing condition corresponding to the object (which will be referred to as an adjustment value, i.e., a distance to an object to be focused, for example, represented by a unit of diopter [dptr]=[1/meter]) and convergence value (which is a distance at which a right sight line and a left sight line cross, for example, represented by metric angle [MW]=[1/meter]). When an object is observed at high magnifying power, an influence due to such discrepancy is remarkable. For example, with ten-power binoculars, the degree of discrepancy is ten times in comparison with the degree of discrepancy of naked eyes. The remarkable discrepancy between the adjustment value and convergence value is a burden to the eyes of the observer and causes the eyes to be fatigued. (It should be noted that the term “convergence” means the visual axes of both eyes which are concentrated when observing an object at a short distance, and the angle formed between both axes is referred to as a “convergence angle”).
In view of the above-described problem, in order to reduce the burden to the eyes when observing an object at a short distance, binoculars provided with a convergence value (convergence angle) compensating mechanism have been developed. In such binoculars, in accordance with the adjustment value, the convergence value (or convergence angle) is adjusted by moving both objective lenses in the direction orthogonal to the optical axes thereof to make the objective lenses located close to each other when observing an object at a short distance. Examples of such binoculars are disclosed in Japanese Patent Publications No. 3090007, No. 3196613 and No. 3189328. However, the structure of a convergence value compensating mechanism of the binoculars described in each of the patent publications is relatively complicated.
For example, the mechanism shown in FIG. 8 of publication No. 3196613 is configured such that the objective lens is moved along two upper and lower guide rods and an auxiliary rod. In this mechanism, the guide rods and auxiliary rod should be prepared separately from a lens frame of each objective lens and implemented in the lens frame. In such a configuration, the number of components is increased, manufacturing and assembling thereof are relatively difficult, and thus the manufacturing costs increases. In addition, since each of the guide rods and auxiliary rod is straight, the inclination angle should be made constant. Therefore, it is difficult to optimally compensate for the convergence value in accordance with a focusing operation.
In the mechanism shown in FIG. 4 of publication No. 3196613, the objective lens is supported in the objective lens frame so as to be movable in a direction perpendicular to the optical axis direction, and the objective lens frame is movably supported in the lens barrel. In the mechanism, it is necessary to provide at least a triple structure in which the lens frames, objective lens frame and lens barrel are employed, thereby resulting in complication and upsizing.
The mechanism shown in FIG. 8 of publication No. 3090007 compensates for the convergence value by moving a prism using a cam. The structure requires, however, two separate actuation mechanisms, including an actuation mechanism for focusing. Therefore, the structure is complicated.